The Salisbury convention — agreed between Lord Salisbury, leader of the Conservatives in the Lords at the time, and Lord Addison, his Labour opposite number after the 1945 general election which gave Labour a landslide majority in the Commons while only 16 peers took its whip — means that the House of Lords does not vote down legislation which is set out in the governing party’s election manifesto. With the Brexit Bill not only being the centrepiece of the Tories’ 2017 manifesto, but also more people in the country having voted for Brexit in 2016 than have ever voted for anything else, it seems unthinkable that the Lords would vote the whole thing down.

Yet the Liberal Democrats — who have 100 peers — have made noises about not being bound by the Salisbury convention as they were not party to it back in the 1940s, perhaps not the strongest possible argument as they have broadly followed it over the intervening 70 years. Some peers will undoubtedly oppose the Bill’s Second Reading, but not enough to prevent its passage. The trouble will begin during the Bill’s Committee and Report stages.

In the Commons government bills are subject to programme motions, which set out a fixed timetable for a Bill’s Committee, Report and final Third Reading stages. The vote on these programme motions is far more important than the vote on Second Reading which immediately precedes it — as without a programme motion a minority of MPs can use a multitude of delaying techniques to stop a piece of legislation reaching the statute books. This can frequently be seen in the Commons in the passage of Private Members’ Bills, many of which never become law — not because there is not a parliamentary majority for them but because a minority of MPs have filibustered them.

In the Lords there are no programme motions. This means that opponents of Brexit will be able to use all manner of delaying tactics to make sure that the passage of the Bill will seem like unending trench warfare. Furthermore, anti-Brexiteers will argue that, while there was a general commitment to Brexit in the Conservative manifesto, there was little detail about the exact practicalities of the process. They will thus feel at liberty to amend the Bill to ribbons.

The government will lose votes during Committee stage, not by the four it lost on Grieve’s amendment in the Commons, but in all likelihood by margins of 60 or 70. How can it respond?

It could create many more Conservative peers, but Theresa May has been reluctant to go down this path — it has taken long enough to draw up the first list of 20 or so new Tories — and time is running out. There is talk of the Lords changing its standing orders to prevent more than one new peer being introduced per day to counter the threat of a Brexiteer flood.